In power plants such as a thermal power plant and a nuclear power plant, steam valves such as a steam control valve are installed in a steam turbine system in order to adjust a flow rate of steam flowing into an inlet of a steam turbine.
FIG. 15 is a cross-sectional view schematically illustrating an example of a steam valve according to a related art.
As illustrated in FIG. 15, a steam valve V10J has a valve casing 21 (valve box), an upper cover 22, a sleeve 23, a main valve 25 (valve body), a valve seat 26, and a valve rod 27, and adjusts a flow rate of steam flowing into an inlet of a steam turbine (not illustrated).
Concretely, in the steam valve V10J, the valve casing 21 is structured so that the steam flows into an inner space from the inlet (not illustrated) provided on a side portion and the steam flows out from an outlet 21E provided in a lower part. Further, the valve casing 21 has an opening 21K formed in an upper portion. The upper cover 22 is installed on the upper portion of the valve casing 21 to close the opening 21K of the valve casing 21. Further, a through hole 22K is formed in the upper cover 21. The sleeve 23 is a tubular body and its upper portion is supported by the upper cover 22 inside the valve casing 21. The main valve 25, which is a tubular body, houses the valve rod 27 therein and is supported by a lower end portion of the valve rod 27. Further, the main valve 25 is housed in the sleeve 23. The valve seat 26 is provided under the sleeve 23 inside the valve casing 21. The valve rod 27 is installed so as to penetrate through the through hole 22K of the upper cover 22 and the inside of the sleeve 23.
In the steam valve V10J, the valve rod 27 is coupled to a driving device (not illustrated) outside the valve casing 21, and by the driving device (not illustrated) driving the valve rod 27, the main valve 25 is slid in an up and down direction inside the sleeve 23. Here, the driving device (not illustrated) operates according to an opening degree instruction signal output from a control device (not illustrated). By the operation of the driving device (not illustrated), an opening degree between the main valve 25 and the valve seat 26 is adjusted and the flow rate of the steam is controlled.
Concretely, inside the valve casing 21, when the main valve 25 moves upward to separate from the valve seat 26 and a gap between the main valve 25 and the valve seat 26 is opened, the steam flowing into the inside from the inlet of the valve casing 21 flows out from the outlet of the valve casing 21. On the other hand, when the main valve 25 moves downward to come into close contact with the valve seat 26 and the gap between the main valve 25 and the valve seat 26 is closed, the flow of the steam is stopped.
By thus controlling the operation of the steam valve V10J, operations such as a governor free operation and a load limiter operation are performed. Note that the governor free operation means an operation in which a governor is freely operated according to a frequency change without a load limiter limiting the governor operation. In the governor free operation, the control is automatically performed so that an output of a rotary machine increases when the frequency decreases (the rotation of a power generator decreases), and the output decreases when the frequency increases (the rotation of the power generator increases).
In a steam turbine system, in order to improve turbine efficiency, it is required to reduce a pressure loss by operating a steam valve with the maximum opening degree. Further, in order to similarly improve turbine efficiency, it is required to reduce the leakage of steam from the steam valve during the operation.
Therefore, in order to prevent the steam from leaking from the gap between the upper cover 22 and the valve rod 27 when the steam valve V10J is opened to the maximum opening degree, improvements have been made in the steam valve.
FIG. 16 is a cross-sectional view schematically illustrating another example of a steam valve.
As illustrated in FIG. 16, in a steam valve V10, a bush 22b is installed in a through hole 22K of an upper cover 22 as is not the case in FIG. 15. Further, a valve rod 27 is installed so as to penetrate through the bush 22b in the through hole 22K of the upper cover 22. Further, in the valve rod 27, a contact surface 27S is formed so as to come into mechanical contact with the bush 22b provided in the upper cover 22 when the steam valve V10 is opened to the maximum opening degree. A projecting portion is formed on an outer peripheral surface of the valve rod 27 so as to project in a convex shape, and in the projecting portion, the contact surface 27S of the valve rod 27 is provided in a portion facing the upper cover 22. Here, the projecting portion has an inclined surface inclined with respect to an up and down direction so as to be larger in outside diameter as it goes from an upper side toward a lower side, and the inclined surface is provided as the contact surface 27S. Further, the bush 22b also has an inclined surface similar to the contact surface 27S of the valve rod 27, and is formed so that the contact surface 27S of the valve rod 27 comes into contact with the inclined surface.
However, when the governor free operation is performed by using the steam valve V10 illustrated in FIG. 16, a system frequency changes while the steam valve V10 is fully opened, and an opening degree instruction signal periodically varies, so that, in the steam valve V10, there sometimes occurs a case where the aforesaid mechanical contact between the bush 22b and the contact surface 27S of the valve rod 27 is repeated. This sometimes causes a crack of their contact portions due to fatigue failure.
As a result, it sometimes becomes difficult to safely operate the steam turbine.